High-speed technology that uses twisted-pair cabling (whether unshielded of shielded) may be susceptible to common-mode noise, which is generated in the environment (e.g., from cell phones and radios). The noise may be picked up by the network cables acting like antennae, or conducted through noise on system grounds, which find their way into the cable via the center tap of the data transformer (or through magnetic coupling along the channel). The noise coupled onto each wire in the cable is then converted to a differential voltage (Vdnoise). This conversion can be referred to as C2D (common-to-differential). Additionally, as the common-mode signal travels down the cable, it can be reflected due to impedance discontinuity in connectors and cables.
Signal integrity hazards contribute to Vdnoise, which mixes with the traveling differential data along the cable. Vdnoise may interfere with the recovery of differential data transmitted on each pair. An Ethernet cable, for example, typically has four pairs, and each pair is driven in a differential manner. While the noise in this case starts in a common-mode fashion, the imbalances in the channel convert the noise to a differential signal and cause data corruption and link impairments in some cases. Differential signaling may, for example, be occurring at ±1 Volt. Theoretically, the noise is coupled from a source onto all pairs in a cable, but even a single pair with common-mode noise on it can deliver the common-mode noise along with a differential error noise onto the other pairs in the cable/connectors. Connectors are a large issue since they are not necessarily designed to work at high frequencies, where imbalances in the connectors are the main cause of C2D conversion.